1. Field of the Invention
This invention relates to turbine lubrication systems, and more particularly, to means for ensuring turbine lubrication during turbine operation at any speed.
2. Description of the Prior Art
Typical lubrication systems for turbines used in large central station electrical generation facilities include a centrifugal pump coupled to the turbine's rotor which discharges lubricant into a line directed to a fluid ejector located in or near a lubricant (usually oil) reservoir. During the lubricant's passage through the ejector from its inlet port to its outlet port, a partial vacuum is created through a suction port in the ejector through which lubricant is drawn from the lubricant reservoir and is mixed with the pumped lubricant. The total volume of lubricant (transmitted by the pump and drawn through the suction port) is discharged through the ejector's outlet port and is routed to the turbine bearings and the suction side of the main lubricant pump. A substantial discharge pressure from the main lubricant pump is required (approximately 350 psi) and a sufficient flow volume (typically approximately 1,000 gallons/min. but dependent on the number and size of the bearings) of lubricant is necessary to provide sufficient vacuum through the ejector's suction port to pick up the additional lubricant and transmit the total volume of lubricant at approximately 15 psi. to the turbine bearings and the main lubricant pump's suction side.
Centrifugal pumps (normally used as main lubricant pumps) have dynamic characteristics which provide insufficient lubricant flow rate and discharge pressure below approximately three-fourths design speed of the turbine to withdraw the required additional lubricant from the reservoir and discharge the total volume of lubricant to the bearings and main oil pump's suction side. Thus, for turbine start-ups and shutdowns additional lubrication supply apparatus has, heretofore, been required. Common practice for start-ups and shutdowns has been to provide redundant pumps which withdraw lubricant from the reservoir and discharge that lubricant to the turbine bearings. Such redundant pumps have been driven by AC electrical motors and, in some cases, DC electrical motors which primarily act as backup in case of failure of the AC electric motor or AC electric supply. There have been some cases where both the AC-driven and DC-driven pumps have been out of service or otherwise inoperable at times when they were needed. In cases where the main lubricant pump's flow and pressure were insufficient to provide the required lubricant to the turbine bearings due to inadequate turbine rotor speed and the redundant pumps were out-of-service, the turbine bearings were sometimes adversely affected and sometimes suffered damage from the lack of oil supplied thereto. Such bearing damage required bearing replacement and/or refurbishment at substantial cost and turbine downtime.
Since the turbine bearings must be lubricated for substantially the entire time the turbine rotor rotates, it was desired to provide some system that would supply the necessary lubricant to the turbine bearings until the rotor comes to rest or reaches a speed at which the main centrifugal lubricant pump will provide sufficient lubricant flow and pressure.